U.S. patent number 5,442,659 [Application Number 08/142,555] was granted by the patent office on 1995-08-15 for radio communications system with fault tolerant frequency hopping synchronization.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Frederick J. Bauchot, Chia-Chi Huang, Ilan Kessler, Kadathur S. Natarajan.
United States Patent |
5,442,659 |
Bauchot , et al. |
August 15, 1995 |
Radio communications system with fault tolerant frequency hopping
synchronization
Abstract
A method for use in multicellular communication network system
of the type having base stations and a plurality of remote
stations. The method achieves reliable and fault tolerant
synchronization between the stations in a call when a frequency
hopping technique is used. The method includes steps for acquiring
the frequency hop sequence by remote station and tracking the
frequency hop sequence after acquisition in order to stay in
synchronism. The method also provides for recovering from loss of
synchronism and for staying in synchronism.
Inventors: |
Bauchot; Frederick J.
(Saint-Jeannet, FR), Huang; Chia-Chi (Hsinchu,
TW), Kessler; Ilan (Bronx, NY), Natarajan;
Kadathur S. (Millwood, NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
22500299 |
Appl.
No.: |
08/142,555 |
Filed: |
October 22, 1993 |
Current U.S.
Class: |
375/134; 375/137;
375/E1.037; 380/34 |
Current CPC
Class: |
H04B
1/7156 (20130101); H04J 13/00 (20130101); H04L
7/043 (20130101); H04B 2001/71563 (20130101); H04B
2001/71566 (20130101) |
Current International
Class: |
H04L
7/04 (20060101); H04B 1/69 (20060101); H04J
13/06 (20060101); H04J 13/02 (20060101); H04B
1/713 (20060101); H04J 13/00 (20060101); H04B
001/713 () |
Field of
Search: |
;375/1 ;380/34,48 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gregory; Bernarr E.
Attorney, Agent or Firm: Perman & Green
Claims
What is claimed is:
1. In a multicellular communications network system comprising base
stations having different addresses and a plurality of remote
stations incorporating a technique for frequency hopping wherein
said base stations transmit information to remote stations using a
sequence of frequency hops of different carrier frequencies,
a method for generating and transmitting consecutive frames of data
for synchronization during said transmission comprising the steps
of:
Step 1 generating frames of data to be transmitted from each of
said base station to said remote stations, said frames including a
header portion and a data transfer portion,
Step 2 incorporating frequency hopping control information into
said frame header portion by incorporating the address of said base
station within the network, incorporating the time remaining in
said frequency hop being used, incorporating the frame length of
said frame and incorporating a list of the next hopping frequencies
to be used in said sequence of changing frequency hopping sequence
tracking technique, wherein said remote station switches to a given
frequency for a given period of time equal to or greater than the
length of a frame, and wherein said remote station monitors the
header information of frames transmitted from a series of other
bases of said network and selects another base as a new home base
when paid SYNC state of operation is not achieved with a selected
number of attempts,
Step 3 transmitting a plurality of said frames from said base
stations to said remote stations using a sequence of frequency hops
having different carrier frequencies.
2. A method according to claim 1 further including a step 4 wherein
a remote station performs a technique for acquiring said plurality
of frames transmitted from said base station in Step 3 by listening
to frames of information transmitted from different bases by
repeatedly listening to different frequencies for fixed periods of
time equal to a frame length, and selecting a home base from said
different bases listened to by locking into the frequency hopping
sequence of said selected base.
3. A method according to claim 2 wherein said remote station
monitors and records indicators from said transmitted frames from
said different bases, said indicators being the received signal
strength indication (RSSI), the header observation rate (HOR) and
the load factor (LF) from said different bases, and wherein said
selected home base is selected in accordance with said indicator
criteria.
4. A method according to claim 3 including a synchronization
technique wherein, after selecting a home base in said acquiring
step of claim 3, said method performs a frequency hopping sequence
tracking technique, said frequency hopping sequence tracking
technique including the steps of acquiring the said frequency
hopping sequence including the next frequency hop, seeking the
header information in the next frequency hop transmitted by said
selected home base, upon finding the header information said remote
station enters a SYNC state of operation, upon not finding the
header information after seeking for a predetermined number of
frequency hops, said remote station enters an OUT OF SYNC state of
operation.
5. A method according to claim 4 wherein, when said remote station
is in said OUT OF SYNC state in said frequency hopping sequence
tracking technique, said remote station switches to a given
frequency for a given period of time equal to or greater than the
length of a frame, and wherein said remote station monitors the
header information of frames transmitted from a series of other
bases of said network and selects another base as a new home base
when paid SYNC state of operation is not achieved with a selected
number of attempts.
6. A method according to claim 5 further including a technique for
fault tolerant frequency hopping synchronization wherein a
frequency hop table is initialized and stored in a memory table by
storing the separate frequencies in the said frequency hop
sequence,
listening at said remote station for a specific frequency referred
to as a default frequency
updating said memory table based on changes in the frequency hop
sequence data in said frame headers transmitted from said base
stations.
Description
FIELD OF THE INVENTION
The present invention relates to communication systems, and more
particularly to synchronization of frequency hopping communication
systems which are fault-tolerant in the presence of transmission
errors.
BACKGROUND OF THE INVENTION
Frequency hopping is a radio communication technique in
spread-spectrum modulation wherein information is transmitted using
a sequence of carrier frequencies that change at set times to
produce a narrow band signal that bounces or hops around in center
frequency over the available spectrum.
In a centrally controlled multicellular mobile radio communication
system based on slow frequency hopping, each cell has a base
station that provides the necessary timing and control information
received and used by all the remote stations that belong to the
cell.
All stations belonging to a cell, the base station and all remote
stations that belong to it, must hop in synchronism in order to
communicate with each other at the same frequency. Different cells
will typically operate on different frequency hopping patterns, The
control information required for synchronized frequency hopping is
broadcast by the base station. A key problem in the operation of a
frequency-hopping based system is that of maintaining hop
synchronization between all stations that belong to the same cell.
Synchronization must be ensured even under conditions of loss of
transmission of control information. The problem of maintaining
synchronism can be further divided into the subproblems of: a)
Obtaining initial synchronism, b) Staying in synchronism and c)
Reacquisition of synchronism after temporary loss of synchronism.
The base station may modify at any time the frequency hopping
pattern (for instance to overcome interferences) and the remote
stations must be able to follow this pattern change in an efficient
and reliable manner.
The following references are typical of the background art in the
field of frequency hopping systems and synchronization techniques
therefor.
In U.S. Pat. No. 5,130,987 issued Jul. 14, 1992 to Flammer entitled
"Method For Synchronizing A Wide Area Network Without Global
Synchronizing", a frequency-hopping packet communication system
without a master clock or master control unit is described which is
based on use of a receiver's frequency hopping timing and
identification to control communication. A frequency-hopping band
plan, involving the number of channels and the pseudo-random
pattern of frequency change and nominal timing of changes, is
universally known to each node in the network. A transmitter
acquires synchronization with a target node by use of information
previously received from or about a target indicating timing of
present idle frequency hop of the target receiver. Each receiving
node establishes in each station or node a table of receiver
frequency hopping sequence offsets (hop timing offsets) of each
other node within its communication range, and each node announces
its communication range, and each node announces its presence on
each frequency in a packet with a hop timing offset indicator. The
hop timing offset indicator is a key used to read a table to allow
nodes to set themselves in synchronization with one another. A
location indicator built into the address of each packet is used to
randomize an ordered frequency-hopping table at each node.
Frequency-hopping is implemented by the division of communication
slots and the accumulation of slots into epochs, wherein each epoch
equals the total number of available slots (number of channels
times the number of time frames per channel). The transmitting node
tracks the pre-established frequency-hopping pattern for its target
receiver based on previously-acquired information.
U.S. Pat. No. 5,121,408 issued Jun. 9, 1992 to Cai et al. entitled
"Synchronization For Entry To A Network In A Frequency Hopping
Communication System" discloses techniques for synchronization of a
frequency hopping transceiver to a network by embedding
synchronization codes in the pseudo-random frequency hopping
transmission sequence. A receiver is implemented with a frequency
detector and a correlator to generate a correlator signal in
response to the synchronization codes in the pseudo-random
frequency detector and a correlator to generate a correlator signal
in response the synchronization codes in the pseudo-random
frequency hopping transmission sequence. Detection of a peak in the
correlator signal is indicative of synchronization of the receiver
with the network. The network entry synchronization scheme is such
that, when two transceivers A and B are communicating, a third
unnetworked transceiver C extracts the hidden network entry code
pattern from the A-B transmission in order to enter the network. As
a part of the communication between the two transceivers A and B,
transceiver A transmits a known pattern as a hidden part of the
communication which allows transceiver C to enter the A-B network.
This hidden code pattern permits rapid synchronization and
correction of large initial time errors, and permits correction of
time drift from then on.
U.S. Pat. No. 5,081,641 issued Jan. 14, 1992 to Kotzin et al.
entitled "Interconnecting And Processing System For Facilitating
Frequency Hopping" discloses a method and apparatus for
facilitating communication of information in a system without the
use of a baseband hopping unit, by sharing a common TDM bus between
a plurality of radio communication units, processing units, and
information links, where the processing units extract traffic
channel information, packetize and/or unpacketize the information,
and return same back to the common bus for retrieval by the
information links or radio communication units.
U.S. Pat. No. 5,079,768 issued Jan. 7, 1992 to Flammer entitled
"Method For Frequency Sharing In Frequency Hopping Communications
Network" discloses a frequency hopping communications system
wherein frequency-hopping is implemented by the division of
communication slots and the accumulation of slots into epochs,
wherein each epoch equals the total number of available slots
(number of channels times the number of time frames per channel). A
transmitting node tracks the preestablished frequency-hopping
pattern for its target receiver based on previously-acquired
information. The transmission node identifies a receiver node. The
transmission node then checks the frequency channel to determine if
available (e.g., not in use and within an acceptable noise margin).
If unavailable, the transmission node delays transmission to the
identified node to a later slot. During the delay, the transmission
node identifies another receiver node and a corresponding current
frequency channel. The steps of identifying a receiver node and
checking the corresponding current frequency channel are repeated
until a node having an available frequency channel is identified.
The transmission node then sends a packet to the selected receiver
node at a frequency and for a duration defined according to the
current slot. Such transmission node tracks the changing frequency
of the selected receiver node to maintain frequency
synchronization.
In U.S. Pat. No. 4,850,036 issued Jul. 18, 1989 to Smith entitled
"Radio Communication System Using Synchronous Frequency Hopping
Transmissions" a frequency-hopping radio communication system is
disclosed comprising a control unit which transmits to and receives
from each of a plurality of slave stations using a
frequency-hopping mode of operation. During a start-up mode, the
control unit communicates a starting message to each slave station
using a predefined frequency. The message identifies to each slave
station a frequency-hopping sequence to be used to select the
frequencies from a group of frequencies for transmission to and
reception from the control unit. This message also specifies to
each slave station unique starting frequencies in the
frequency-hopping sequence at which to begin transmitting and
receiving. All slave station transmission are synchronized to the
control unit transmissions, thereby preventing any two stations
from concurrently using the same frequencies for either
transmitting to or receiving from the control unit.
In U.S. Pat. No. 4,612,652 issued Sep. 16, 1986 to Kadin entitled
"Frequency Hopping Data Communication System" an improved frequency
hopping data communication system with a random transmission
bandwidth to provide independent frequency hopping of the mark and
space frequency is provided in the system which is particularly
immune to repeater jamming. Only one frequency is transmitted at a
time upon selection on a bit instant by a pseudo-noise code
generator. The location of the mark and space frequency is randomly
chosen, however, the location is known at the transmitter and the
repeater by appropriate synchronization equipment.
SUMMARY OF THE INVENTION
In accordance with the present invention, methods and structure are
provided for achieving reliable and fault tolerant synchronization
between the stations within a radio communication cell in the
presence of errors.
The present invention enables a remote stations to acquire an
initial hop pattern after a station is powered on and to track the
hopping patterns after it is initially acquired in order to stay in
synchronism as long as it is powered on and the base station is
operable. The present invention also provides for the recovery of a
station from loss of synchronism that may be caused by transient
propagation conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a pictorial diagram showing a typical radio digital data
communication system of the type in which the invention is
implemented;
FIG. 1A is a block diagram of the system shown in FIG. 1
illustrating the basic components of a mobile station and a base
station as known in the art;
FIG. 2 is a block diagram of the radio system used in the
implementation of a preferred embodiment of the invention;
FIG. 3 is a schematic illustration of a time frame of a frequency
hop used in a communication system according to the present
invention;
FIG. 4 is a schematic illustrations of the information included in
the header of the time frame shown in FIG. 1;
FIG. 5 is an illustration of a flow chart of steps employed in the
monitoring and selection phases of the synchronization technique of
the present invention;
FIG. 6 is an illustration of a flow chart of steps employed in the
tracking phase of the synchronization technique of the present
invention;
FIG. 7 is a flow chart of the steps employed for locking into a
hopping pattern and tracking the hopping pattern in a
synchronization technique according to the present invention;
FIG. 8 is an illustration of a flow chart for a memory table based
approach for maintaining frequency hopping synchronization
according to the principles of the present invention; and
FIG. 9 is an illustration of a frequency hop with multiple
frames.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, and more particularly to FIG. 1,
there is shown a typical radio system allowing communication
between a plurality of mobile stations 10, 12, 14, and 16 and
applications and data residing in a computing system. The computing
system typically includes a Wireless Network Manager (WNM) or
Wireless Network Controller 18, with attached monitor 20 and
keyboard 22, of a local area network (LAN), generally indicated by
reference numeral 24, having a plurality of attached workstations
or personal computers (not shown for simplicity). Also attached to
the LAN are one or more gateways 26 and 28 with which the mobile
stations 10, 12, 14, and 16 communicate. These gateways, referred
to as base stations, are augmented according to the invention to
provide certain radio system management functions which coordinate
the mobile stations' access to the common radio channel.
Communications between mobile stations is supported via relay
through the base stations 26 and 28.
shown in more detail in FIG. 1A, a base station 26 or 28, which may
be a conventional microcomputer, has a LAN adapter 30 inserted in a
bus slot and connected to LAN cabling 32. The WNM 18, typically
also a conventional microcomputer and including one or more direct
access storage devices (DASDs) such as hard disks (not shown), also
has a LAN adapter 34 inserted in a bus slot and connected to LAN
cabling 32. The LAN adapters 30 and 34 and LAN cabling 32 together
with LAN software constitute the LAN 24. The LAN 24 is of
conventional design and does not form part of the invention. The
base station 26 or 28 also has an RF transceiver adapter 36
implemented as a printed circuit card which is inserted in a bus
slot of the base station. The transceiver adapter 36 includes a
spread spectrum transceiver of conventional design. The transceiver
adapter 36 has an antenna 38 by which a radio link 40 is
established with one or more remote or mobile stations, 10, 12, 14,
or 16. The mobile station may itself be a hand held or lap top
computer of conventional design and, like the base station, it is
provided with an antenna 42 and a transceiver adapter 44, also
implemented as a printed circuit card which is inserted in a bus
slot of the computer. The transceiver adapter 44, like transceiver
adapter 36, includes a spread spectrum transceiver of similar
design. The base station and the mobile stations are further
provided with software, generally indicated by reference numerals
46 and 48, respectively, which support their respective transceiver
adapters.
FIG. 2 shows the radio system common to both the mobile stations
and the base stations of FIG. 1. The radio system includes a
transceiver adapter 36 or 44 connected to the computer 50 via the
computers bus interface 52. The transceiver station is itself
divided into an RF transceiver 54, which may be a commercially
available spread spectrum transceiver, and a dedicated
microprocessor system 56 which controls the transceiver via an
interface 58. The microprocessor system 56 further includes a
system interface 60 which interfaces the transceiver section to the
computer section 50. The microprocessor system includes a dedicated
microprocessor 62 containing high-resolution time interval
determination hardware or "timers" typical of real-time
microprocessor systems.
Microprocessor 62 is connected by a memory bus 64 to program
storage 66 and data storage 68 as well as to interfaces 58 and 60
providing attachment to bus interface 52 and RF transceiver 54,
respectively. Program storage 66 is typically read only memory
(ROM), while data storage 68 is static or dynamic random access
memory (SRAM or DRAM). Packets received or to be sent are held in
data storage 68 and communicated to or from the RF transceiver 54
via interface 58 under control of serial channels and a direct
memory access (DMA) controller (not shown) which is part of the
microprocessor 62. The function of these serial channels is to
encapsulate data and control information in an HDLC (high-level
data link control) packet structure and provide the packet in
serial form to the RF transceiver 54. For more information on the
HDLC packet structure, see, for example Mischa Schwartz,
Telecommunication Networks: Protocols, Modeling and Analysis,
Addison-Wesley (1988).
When a packet is received through the RF transceiver 54, the serial
channels check the packet destination address, check for errors,
and deserialize the packet to data storage 68. The serial channels
must have the capability to recognize a specific adapter address as
well as a broadcast address. Specific microprocessors with
appropriate serial channel and timer facilities include the
Motorola 68302 and the National HPC46400E microprocessors.
The computer 50 runs an operating system 70 which supports one or
more user application programs 72. The operating system 70 may
include a communications manager 74, or the communications manager
74 may itself be an application program installed on the computer.
In either case, the communications manager 74 controls a device
driver 76 via the operating system 70. The device driver 76, in
turn, communicates with the transceiver adapter 36 or 44 via bus
interface 52.
Referring to FIG. 3, an illustration showing one time frame of a
sequence of frequency hops including a control header AH and a
plurality of slots with a data transfer phase A as employed in the
present invention.
For the purpose of explanation, the embodiment of the invention
will have a hop consisting of exactly one frame.
During the control phase the frame header AH containing control
information is broadcast by the base station. The control
information within AH includes among other data, the information
necessary for stations to perform frequency-hopping and stay in
synchronism with respect to each other.
The data transfer phase A includes the outbound data transmitted
from the base station to mobile stations, and inbound transfer of
data transmitted from the mobile stations to the base station in
accordance with a multiple access protocol.
The aforesaid control information within the AH header necessary
for stations to perform frequency hopping is shown in FIG. 4. The
addressing information contained in header AH as shown in FIG. 4
includes all pertinent information required to uniquely identify a
base station. For example, this could consist of a pair
<NETWORKID, BASEID> where NETWORKID is the identification of
the network and BASEID is the identification of the base station
within the network. TIMEREMINHOP is a parameter indicating the
remaining time duration of this hop. A remote station will use this
information to determine when to switch to the next carrier
frequency of the hop pattern. FRAMELENGTH is a parameter indicating
the length of the frame. This information is used by remote
stations to determine when to expect the AH header information in a
sequence of frames. F(l), . . . , F(N) are the next N hopping
frequencies. The list of N frequencies are received and used by
remote stations for the following purposes:
The list of frequencies is used by a remote to build up and conform
to the specified hopping pattern.
The list will be used by a remote to update its hopping pattern.
The remote needs to keep track of pattern changes whenever the base
station uses a dynamic hop revision policy for combating
interference in the cell.
The value of N, a system design parameter, is chosen such that the
remote station can maintain synchronism with a high degree of
reliability. Assuming N=4 and Header AH error rate is less than 1
percent, the probability of loss of all 4 consecutive headers will
be less than 10.sup.-8.
Referring to FIG. 5, the initial pattern acquisition steps are
shown. When a remote is first turned on, it does not know who are
the surrounding bases and what frequency hopping patterns they
have. However, it is assumed that it knows both the hop length and
the superframe length. A remote depends on executing the algorithm
shown in FIG. 5 in selecting its home base. When a remote is first
powered up, it listens at a fixed frequency and searches for valid
header messages from neighboring bases. The indicators such as RSSI
(received signal strength indication), HOR (header observation
rate), and LF (load factor) can be monitored. After a fixed period
of time which is equal to the length of a superframe, it switches
to another frequency and keeps on monitoring. During this
monitoring process, the remote keeps records on RSSI, HOR, and LF
from each base. A number (M) of frequencies are examined before a
remote chooses an initial home base because the remote should
depend on average RSSI observed at several frequencies to eliminate
the effect of frequency dependent fading. Besides, HOR and LF are
parameters which are more meaningful when they are calculated by
averaging the results of several monitoring cycles.
In selecting an initial home base, different emphasis can be placed
on the selection criteria, RSSI, HOR, or LF. Methods for selecting
an initial home base based on multiple selection criteria are known
in the art. After a fixed number of (e.g., M=5) frequencies have
been examined and an initial home base is selected, the remote
locks into the frequency hopping pattern of its chosen home
base.
After a remote initially acquires the frequency hopping sequence
from its home base, it enters into a hopping pattern tracking
phase. In this phase, the flow chart illustrated in FIG. 6 is
executed.
The remote is in a "SYNC" state after it acquires the frequency
hopping sequence. At that point, the remote tries to look for the
header message in the following frequency hop. If it finds it, it
stays in the "SYNC" state. Otherwise, it enters the "MISS 1" state.
At this point, it tries to look for a header again in the next
frequency hop. If it finds it, it returns to the "SYNC" state.
Otherwise, it enters the "MISS 2" state. This procedure of header
hunting continues until N header in a sequence are missing. In this
situation, the remote enters an "OUT OF SYNC" state.
At the "OUT OF SYNC" state, the remote switches to a certain
frequency for a prolonged period of time (at least the length of a
superframe) and monitors the header transmissions from all the
neighboring bases. If it finds the header from its home base, it
gets back to the "SYNC" state again. Otherwise, it switches to
another frequency and monitors the headers. This process of header
monitoring ends when after monitoring M frequencies the remote does
not find a header from its home base. At this point, the remote
chooses another home base and locks into its frequency hopping
pattern.
The method described above enables a remote station to keep track
of its hopping pattern and maintain fault-tolerant F
synchronization if more than 4 consecutive frames are missed. The
logic is summarized in the flowchart shown in FIG. 7. In the
flowchart, AH.F (1), AH.F (2), AH.F (3) and AH.F (4) correspond to
the four frequencies indicated in each AH header. TF is the
duration of a frame (=hop length/number of frames per hop). The
four RAM positions f(1), f(2), f(3) and f(4) are defined to give
the next four frequencies to be used after the current
frequency.
A fault tolerant frequency hopping synchronization based on a
memory table based approach will next be discussed. The main idea
of this approach is to learn the frequency hopping pattern,
memorize it and update it if necessary, using the steps illustrated
in FIG. 8. Then if successive frequencies are missed, then the
memorized table is used. Suppose N corresponds to the number of
frequencies in the frequency hopping pattern. Let N RAM positions
f(1), f(2), . . . , f(N) give the sequence of frequencies in the
FHP. The RF modem maintains a hop table. The modem first listens at
a default frequency, fdefault. The hop table is maintained current
by updating it based on the F pattern information in Header AH.
Dynamic changes to the hopping pattern are conveyed by the base
station in every header and used by remote stations to update their
F table. The only case where this scheme fails corresponds to the
situation where a frequency is changed just after a sequence of
four consecutive missed frequencies. Nevertheless, if the following
frequency matches with the memorized one, then the synchronization
is ensured again. Hence, the scheme is very robust.
If a frequency hop contains more than one frame as shown in FIG. 9,
then the implementation can be generalized in a straightforward
manner to achieve hop synchronization described above.
What has been described is a method for periodic broadcasting of
Frequency Hopping control information from a base station to a set
of remote stations that wish to hop in synchronism with the base
station. The method also provide for the acquisition o Frequency
Hopping information by a newly arrived remote station (i.e., one
that has been just turned ON) and for reliable and continuous
tracking of the Frequency Hopping information even when the
broadcast control header messages from the base station may
intermittently be lost due to poor propagation conditions.
While the invention has been described in connection with a
preferred embodiment, it is not intended to limit the scope of the
invention to the particular form set forth, but, on the contrary,
it is intended to cover such alternatives, modifications, and
equivalence as may be included within the spirit and scope of the
invention as defined in the appended claims.
* * * * *